Greul beat Brazilian qualifier Thiago Alves 6-2, 7-5, Devvarman ousted local player Raven Klaasen 6-4, 6-3, and De Voest battled past Stefano Galvani of Italy to win 6-3, 5-7, 7-6 (6).
Only three singles matches were scheduled for the opening day of the ATP event at Montecasino hotel and entertainment complex.
De Voest held off a fightback from Galvani to eventually win in 2 hours, 40 minutes, the South African's first victory on the ATP World Tour in two years.
Greul and Devvarman then cruised to straightforward victories.
Defending champion Feliciano Lopez of Spain, the No. 1 seeded player and highest ranked in Johannesburg at No. 33, plays Canada's Frank Dancevic in the first round on Tuesday.
Second-seeded Yen-hsun Lu of Taiwan, ranked No. 37 and the only other top-50 player at the tournament, will open his campaign against Canadian qualifier Milos Raonic.
Also Tuesday, third-seeded Janko Tipsarevic faces Jesse Huta Galung of the Netherlands and fourth-seeded Kevin Anderson of South Africa takes on Switzerland's Stephane Bohli.